Doctor Who episode 207: The Wheel in Space – Episode 4 (18/5/1968)
Based on the evidence of this episode, The Wheel in Space is a prequel to both The Moonbase and The Tomb of the Cybermen, with the Wheel’s crew apparently completely ignorant of the existence of the Cybermen – and even of their invasion of Earth in 1986. This prompts the Doctor to give a fairly effective potted history:
They were once men, human beings like yourself, from the planet Mondas, but now they’re more robot than man… Their entire bodies are mechanical and their brains have been treated neuro-surgically to remove all human emotions, all sense of pain. They’re ruthless, inhuman killers.